1. Field Of The Invention
The present invention relates to a method for fabricating ballasted finger electrodes.
2. History Of The Art
Finger electrodes comprising a plurality of long, narrow conductive elements extending generally transversely from a common conductive connecting element are used in a variety of electronic devices including microwave and ultrasonic devices. Such electrodes are used, for example, in overlay-type microwave power transistors. Such transistors typically utilize an emitter metallization pattern comprising finger electrode structure with the common connecting element connected to a bonding pad at one side of a base pocket and a plurality of thin, narrow, elongated "fingers" extending across successive rows of emitter sites formed in the base pockets. The base metallization pattern can also comprise a second finger electrode structure interleaved with the first and connected to a bonding pad on the opposite side of the base pocket.
Because the thickness and width of individual fingers are very small, the cross-sectional area is difficult to control with high precision. As a result, there can be substantial variation of electrical resistance from one finger to another.
In order to minimize variations in resistance from one finger to another, the fingers are usually "ballasted" by providing resistive regions in each finger. Such regions act as dominant series resistances and substantially reduce the percentage variation in resistance among the fingers. The usual method of fabricating ballasted finger electrodes on a semiconductor device involves disposing thin resistive layers on the device, depositing the electrode metal on the resistive layer, and etching the metal to define each finger electrode as two or more spaced apart conductive segments interconnected by an underlying resistive layer.
The difficulty with this approach is that the etching process tends to undercut the metal in a manner which is essentially uncontrollable with the result that the spacing between electrodes, and hence the length of the resistive path, is difficult to control. Accordingly, there is a need for a more accurate and more controllable process for fabricating ballasted finger electrodes.